A differential pH/thermal titration apparatus has been developed which permits the study of small and rapid temperature changes associated with the exothermic heats of reaction of acid or base titration of protein solutions. Simultaneous measurement is made of the associated pH changes. Two reaction cells are maintained at constant temperatures (.005 degrees C drift in 2000 seconds) in a common heat exchanger. Acid or base is delivered at a constant rate and rapid mixing is accomplished with a constant-speed (plus or minus .5%) stirring system. pH and temperature changes are recorded on a single cell or differential basis.